The present invention relates generally to mechanical jacks used for raising heavy objects and, more specifically, to a screw-operated scissor jack having an expanded range of load lifting capabilities provided by use of urethane or similar material as a braking means and having faster, smoother operation made possible by use of a thrust bearing that includes a plurality of ball or roller bearings.
Screw-operated scissor jacks have long been known to be useful in lifting applications and especially in situations where it may be desired to level heavy objects. A particular type of well known screw-operated scissor jack employs a double lead Acme screw which traditionally has proven to be particularly advantageous where extremely massive objects need to be raised quickly. One industry in which jacks having the double lead Acme screw have been widely used is the railroad industry, where the need often has arisen to lift locomotives and rail cars from train tracks. For this and similar types of lifting jobs, the double lead Acme screw has been shown to be capable of raising loads up to three times faster than a standard SAE screw that has been used in other jacks.
In addition to providing a faster operating jack, the Acme double lead screw exhibits a further operational advantage that derives from the physical characteristics which are unique to the Acme screw thread. Such operational advantage is the ability for the Acme screw to become self-locking when the jack is subjected to loads generally in excess of one thousand pounds. Where loading is above the stated level, it has been determined that frictional forces developed among the thread lands or roots become sufficiently large to prevent the vertically downward directed force of the lifted object from causing the screw to unwind and prematurely allow the lifted object to descend. As already suggested, the described advantage, which also may be termed an xe2x80x9cAcme loading phenomenon,xe2x80x9d requires that a minimum load be lifted by the jack before the Acme loading phenomenon takes effect and becomes of any benefit to the jack operator. Thus, the advantage to be gained from discovery of a means to lower the minimum load at which the jack will become self-locking has been recognized, and the present invention provides a simple and inexpensive jack construction that is aimed at achieving that end.
In accordance with a preferred embodiment of the present invention, there is provided a screw-operated scissor jack assembly including a double lead Acme screw, used for lifting and on some occasions, leveling a heavy object; the jack assembly being capable not only of raising the object at a faster rate than conventional SAE screws used for the same purposes, but also of becoming advantageously engaged in a self-locking state at loads that are markedly lower than those heretofore required to cause traditionally available scissor jack assemblies to achieve self-locking operation. The jack assembly of the present invention is comprised of: a base member having a plurality of foot-like projections provided for resting the jack assembly against a relatively hard, flat, stationary surface; a first movable arm member rotatably connected at a first end of said first movable arm member to said base member by a first bolt or similar fastening means; a second movable arm member rotatably connected at a first end of said second movable arm member to a second end of said first movable arm member by a first trunnion; a third movable arm member rotatably connected at a first end of said third arm member to said base member by a second bolt or similar fastening means; a fourth movable arm member rotatably connected at a first end of said fourth movable member to a second end of said third movable member by a second trunnion; a first and a second load supporting bracket, each of which brackets is rotatably connected to a second end of each of said second and fourth movable arm members by a third and a fourth bolt or similar fastening means; a rotatable shaft member extending within said first, second, third and fourth movable arm members and having a double lead Acme threaded screw engaged with a threaded bore provided in said second trunnion; and a turning means affixed to an unthreaded end of said rotatable shaft member and located proximate to said first trunnion, said turning means including an operating handle receiver, a thrust bearing, a ring-like braking means comprised of urethane or a similar substance; a first and second washer and a locking pin.
It is therefore an object of the present invention to provide an improved screw-operated scissor jack assembly with a double lead Acme screw, which assembly is operable at high speed and with smooth action.
It is yet another object of the present invention to provide an improved screw-operated scissor jack assembly with a double lead Acme screw, which assembly is operable with a self locking action over a wider range of loads and especially at lower loads in a range of 700-1200 pounds where an Acme loading phenomenon that results in said self locking action has previously been unattainable.